BIZARRE: You've Never Heard Of The Largest Ad Agency In The U.S.

Industry insiders thumbing through Ad Age's 2012 Agency Report, which ranks agencies by revenue size, might do a spit-take with their coffee when they see the name of the largest agency in the U.S.

It's Epsilon, a consulting, marketing data, and technology shop.

Not DraftFCB, not BBDO, not McCann Erickson, not Leo Burnett, and not any of the other usual suspects whose names crop up frequently in the business and consumer media.

Never heard of Epsilon? You're not alone. Despite its massive size it keeps a low profile. It's a unit of Alliance Data Systems, which manages store and branded credit card marketing programs. You probably see Epsilon's campaigns a lot—they're often in the form of permission-based email loyalty programs. Epsilon had $898 million in 2011 revenue, Ad Age reports.

If Epsilon were counted as an agency holding company it would still be the 10th largest in the world. (WPP is No.1, with $16 billion in revenue.)

It gets weirder. You've probably never heard of the No.2 biggest agency in the U.S., either: Acxiom of Little Rock, Ark., which booked $656 million in revenue last year. Like Epsilon, it's a giant cruncher of market research data rather than a traditional creative/media buyer.

DraftFCB is the third largest agency, with $537 million in U.S. revenues, by Ad Age's ranking.

One more surprise from the data: In terms of "consolidated agency networks" (global agencies with many offices under a single name), the No. 1 agency is another brand that will leave most Americans scratching their heads: It's Dentsu, headquartered in Japan.